1. Field of the Invention
The invention relates to the field of combustion technology. It relates to a method of operating a swirl-stabilized burner operated with gaseous and/or liquid fuels and to a burner, in particular a premix burner, suitable for this.
2. Discussion of Background
Swirl-stabilized burners are known. In this type of burner, a swirl generator having a supercritical swirl provides for intensive mixing of fuel and combustion air. The flame stabilization is based on the generation of a backflow bubble, also called backflow zone, which results from the breakdown of the vortex. The ignition of the flame is initiated in front of the stagnation point of this backflow zone, and a stable flame front forms.
An example of swirl-stabilized burners of this type is the burner of the double-cone design, the basic construction of which is described in EP 0 321 809 B1. This premix burner essentially comprises at least two hollow conical sectional bodies, which complement one another to form one body and have tangential air-inlet slots and feeds for gaseous and liquid fuels, in which burner the center axes of the hollow conical sectional bodies have a conicity widening in the direction of flow and run offset from one another in the longitudinal direction. A fuel nozzle is placed at the burner head in the conical interior space formed by the conical sectional bodies. Via gas injectors arranged along the inlet slots, the gaseous fuel is fed to the combustion-air flow prior to its inflow into the burner interior space. The fuel/air mixture is therefore formed directly at the end of the tangential air-inlet slots.
The increase in swirl along the cone axis, in combination with the sudden widening in cross section at the burner outlet, leads to the formation of a backflow zone (inner recirculation zone) downstream of the burner outlet on the burner axis, which backflow zone stabilizes the flame. The ignition of the flame is initiated in front of the stagnation point of this inner backflow zone. As short a backflow zone as possible is aimed at.
If these burners are designed in an unfavorable manner, however, the central backflow zone spreads well downstream on the cone axis into the combustion chamber. A long, almost cylindrical, backflow zone results, the heat being extracted from the backflowing flue gases in the core. Consequently, when the burner is being started, only cold medium is transported back at the stagnation point in front of the backflow zone, at which the flame stabilizes. This has the disadvantage that the ignition of the fuel/air mixture is made more difficult or is even prevented. In addition, the flame stabilization is weakened and the pollutant emissions, in particular carbon monoxide and unburnt hydrocarbons, are increased when the burner is being started.
These disadvantages may also occur in burners having passive flue-gas recirculation by injector delivery, which are described, for example, in EP 0 436 113 B1. There, during the starting action, in addition to the injection of normal combustion air into the vicinity of the injectors, air is injected into the burner head. In the applications realized at present, the result of this is that the surrounding area of the burner head is purged with the cold starting air and the recycling of hot flue gases from the combustion space is impaired. The system heats up only very slowly.
Here too, therefore, the ignition of the fuel/air mixture is made more difficult, the flame stabilization is weakened, and the pollutant emissions are increased.